1. Field of the Invention
The present invention relates to a steel sheet that can be suitably press-worked to be formed into a body part of an automobile, and more particularly to a steel sheet for press working that exhibits excellent stiffness and satisfactory press workability.
2. Related Background Art
As a general rule, a steel sheet for press working must exhibit both excellent ductility (a large E1 value) and satisfactory deep-drawing characteristics (a large r value). In order to cause a steel sheet to have the foregoing characteristics, a method has been usually employed in which the composition of the steel and conditions for the rolling work or heat treatment conditions are controlled at the time of manufacturing the steel sheet. For example, Japanese Patent Laid-Open No. 58-144430 has disclosed a method of minimizing impurity elements, such as C, S and N, that deteriorate the foregoing characteristics.
Recently, there has been a need for the foregoing steel sheet exhibiting the excellent ductility and satisfactory deep-drawing characteristics to further exhibit excellent stiffness. It is well known that stiffness can be improved in proportion to the Young's modulus if the thickness and the molded shape are the same. However, the Young's modulus of the steel sheet cannot be easily controlled to an arbitrary value using the conventional technologies, and therefore, a problem arises in its practicality.
On the other hand, in order to realize the following characteristics, a method of manufacturing a steel sheet for press working that exhibits different characteristics in a direction of the thickness thereof has been disclosed. The foregoing method is an application of a carburizing technology disclosed in, for example, Japanese Patent Laid-Open No. 58-39736, Japanese Patent Laid-Open No. 59-74259, Japanese Patent Laid-Open No. 60-149729, Japanese Patent Laid-Open No. 1-96330, Japanese Patent Laid-Open No. 3-56644, Japanese Patent Laid-Open No. 3-199343 and Japanese Patent Laid-Open No. 3-253543.
However, the conventional technologies disclosed above cannot improve both press workability and stiffness although they are able to improve either of the two characteristics.